1.Field of the Invention
The present invention relates generally to sensing welding process characteristics, and more particularly to the sensing of gas-metal-arc welding process characteristics in order to facilitate control of the welding process, and avoid flaws due to departure from desired process characteristics.
2. Description of the Prior Art
Gas-metal-arc welding (GMAW) is a process in which a consumable electrode is continuously fed into an electric arc. The electrode carries the current powering the arc and provides the filler metal while the arc is the heat source for melting the base metal to be welded and the filler metal added to the weld. The welding current is applied between a contact tube in the vicinity of the arc through which the electrode slides and the workpiece. An inert or slightly reactive shielding gas is used to displace the atmosphere from the arc and the weld pool until solidification occurs, such that the molten metal does not react with the high oxygen and nitrogen levels in the atmosphere. The shielding gas also ionizes to form a high-temperature plasma which carries the current. A mixture of argon with additions of oxygen or carbon dioxide is generally used for welding low alloy steels.
Most GMAW is performed with a constant voltage power source, such that the arc length is self-regulating. If some perturbation causes the arc length to increase, the following steps bring the arc length into equilibrium: the circuit resistance increases; the arc current decreases; the resulting lower current melts the electrode more slowly than the electrode feed rate; and the arc length decreases to a stable length. If some perturbation causes the arc length to decrease, the circuit resistance decreases and the system returns to equilibrium through the opposite sequence.
A more comprehensive description of GMAW is included in National Institute of Standards and Technology Publication No. NISTIR 3976, by Heald, Madigan, Siewert and Liu, entitled "Droplet Transfer Modes for a MIL 100 S-1 GMAW Electrode", published in October, 1991. This publication is hereby incorporated into the present application by reference. As described in detail in NISTIR 3976, metal transfer in GMAW takes place in one of three modes: short circuit (wherein the arc is periodically extinguished and reignited as the advancing electrode contacts the work), globular transfer (where relatively large droplets are transferred at relatively low frequency), and spray transfer (where relatively small droplets are transferred at relatively high frequency). Further, spray transfer may take place in drop spray, streaming spray or rotating spray modes; for the purposes of the present invention, these three modes of spray transfer are essentially equivalent.
For reasons fully described in NISTIR 3976, most production GMAW is carried out in the spray transfer mode. Many GMAW power supplies provide pulses in the welding current and/or voltage, to encourage proper metal transfer.
The art is replete with attempts to monitor various welding parameters to predict and control GMAW, in particular the mode of metal transfer. NISTIR 3976 summarizes many such attempts. As reported in NISTIR 3976, the mode of metal transfer which takes place in any particular welding process is a complex function of at least the electrode feed rate, the arc voltage, the welding current, and the contact tube to work distance (CTWD). Briefly, automated control of GMAW is complicated because those process variables which can be directly measured and controlled, namely, the CTWD, the total voltage between the power supply and workpiece, and the welding current, do not directly reflect the proper performance of the welding process.
More specifically, the CTWD (which can be measured and controlled) is the sum of the arc length and the electrode's extension from the contact tube (or "stickout"). The arc length and electrode extension are of separate relevance to the weld characteristics such as bead height and width, penetration, and undercut, such that measurement of the CTWD does not allow direct control of any of these parameters. Further, the arc voltage, which cannot be directly measured, is but one component of the measured contact-tube-to-workpiece voltage, the other components being the voltage drop between the contact tube and the electrode, and that along the length of the electrode extension. Finally, the inductive nature of the circuit limits the utility of measurement of the welding current as a predictor of the mode of metal transfer. Accordingly, the transfer mode, the electrode extension and the arc length are complex and interdependent functions of the total voltage, the current, the electrode feed speed, and the CTWD.
The contact tube, the electrode extension and the arc are all elements of a GMAW electrical circuit. Changes in the resistance of any element affect the electrical impedance of the circuit. Metal transfer across the arc is characterized by repetitive events, each event modulating the circuit impedance in a characteristic pattern.
As noted, it is well known to use a pulsed power source for enhanced droplet detachment in GMAW. Examples of pulsed GMAW power sources are found in U.S. Pat. No. 3,864,542 to Fletcher et al and U.S. Pat. No. 4,943,701 to Nakajima et al. The signal from such a source exhibits significant current and voltage pulses to stimulate the formation and detachment of droplets at the electrode tip. Such pulsed power sources typically also include internal logic circuitry for changing the pulse frequency along with the wire feed rate. Proper characterization of the various droplet transfer modes and events that interfere with stable transfer might permit voltage or current records derived from sensors used to monitor the arc and to be analyzed to evaluate weld quality in real time and to make corrections as necessary. However, apparatus to do so is not presently available.
Some automated welding systems employ "through-the-arc sensing", that is, monitor the arc voltage and/or current. This technique typically uses a low-frequency sensing strategy, wherein the arc voltage is measured repetitively. The low sampling rate of these conventional systems limits the response time of the welding control system to correct flaws. For example, seam tracking algorithms (that is, the control programs employed by welding systems that automatically follow a joint between two members) look for changes in the mean welding current or voltage (e.g., over a period of several tenths of seconds) to detect departure of the welding unit from the seam.
The system described in the patent issued to Fletcher et al varies the frequency of the DC welding pulses applied to the arc while monitoring the arc voltage to determine the frequency at which the maximum arc voltage is observed. Fletcher et al report that operating the welding power supply at that frequency provides the optimal welding characteristics. However, the Fletcher system is intended for gas tungsten arc welding ("GTAW") (also known as tungsten inert gas, or "TIG", welding), wherein the arc is formed between a nonconsumable tungsten electrode and the workpiece. Accordingly, in GTAW, the arc length is constant; by comparison, in GMAW, as addressed by this invention, the arc length fluctuates in a more complex manner. Accordingly, in GTAW the arc voltage is essentially constant, while in GMAW the arc voltage varies considerably. Hence, the teachings of Fletcher et al are not directly applicable to GMAW.
Ideally, a computer-controlled GMAW system would detect signals from various sensors monitoring the key variables in the welding process, determine the mode of metal transfer, for example, by comparing the monitored variables to preset values, and alter the welding process accordingly by sending control signals to the welding power source. Control actions responsive to the sampled signals would be developed rapidly, so that possible flaws in the welding process might be detected on a real-time basis and the parameters adjusted as necessary to correct the on-going welding process. Only in this manner can automated welding be carried out without an unacceptable number of unsatisfactory welds.
As noted, detection of meaningful characteristics of the welding process is critical in determining whether an ongoing welding process should be altered or terminated. For example, often GMAW is intended to be carried out in the spray transfer mode; the spray transfer mode exhibits characteristic electrical signals from which weld quality information can be obtained. It is likewise important to detect characteristic patterns of electrical signals associated with the short circuit and globular transfer modes. Proper signal processing would allow detection of the transfer mode, droplet transfer frequency, and droplet transfer stability. If welding is being carried out in the short circuit transfer mode, the frequency of the short circuits will indicate if appropriate voltage or current levels are being output by the GMAW power source.
More specifically, as indicated above it would be desirable to provide an automated welding control system capable of monitoring the weld parameters which control the mode of metal transfer from the electrode to the workpiece. As indicated above, metal transfer in GMAW may take place in a short circuiting mode in which the electrode periodically physically touches the workpiece, thus shorting and extinguishing the arc; a globular transfer mode in which the arc is continuous but metal is transferred as a relatively unstable series of large globules; and a spray transfer mode, wherein smaller droplets are transferred at a higher frequency. The spray transfer mode is generally preferred for production rate welding and therefore it is desired to provide a control system which can monitor the welding operation to determine whether spray transfer is occurring and correcting the welding parameters as necessary.
It is understood that the mode of metal transfer is determined at least in part by the arc length. In GMAW, the arc length tends to fluctuate both randomly, for example, due to irregularities in the workpiece causing variation in the CTWD, and periodically, that is, corresponding to detachment of metal droplets from the electrode. Moreover, although the arc length could be determined if the voltage across the arc itself were known, as indicated above in GMAW it is only possible as a practical matter to monitor the voltage supplied between the contact tube and the workpiece. However, the arc voltage, the parameter of real interest, is but one of several voltages which sum to the total voltage supplied, the others being the voltage between the contact tube and the electrode, and the voltage drop along the length of the electrode extension. As the consumable electrode is fed into the arc during the GMAW process, the electrode extension varies quite substantially with droplet detachment and with variation in the CTWD. The voltage drop between the electrode and the contact tube may also vary due to intermittent contact and the like.
It would clearly be desirable to provide a simple and reliable apparatus and method for determining the arc length, such that the arc length could be employed by the control circuitry of a commercial welding apparatus as a direct and objective measure of the welding process, in order to determine whether the arc welding process was proceeding in a satisfactory manner. Typically the arc current and/or electrode feed rate would be varied to control the arc length.
The same control process would be useful in control of other welding processes employing a continuous consumable electrode fed automatically into the region of the arc, for example, flux-cored arc welding, in which the arc may or may not be shielded by an inert gas as in GMAW, but is protected in part by a flux provided in the core of the continuous electrode.